Bile Duct Adenoma with Oncocytic Features

Bile duct adenomas are benign bile duct proliferations usually encountered as an incidental finding. Oncocytic bile duct neoplasms are rare and the majority are malignant. A 61-year-old male with a diagnosis of colorectal adenocarcinoma was undergoing surgery when a small white nodule was discovered on the surface of the right lobe of his liver. This lesion was composed of cytologically bland cells arranged in tightly packed glands. These cells were immunopositive for cytokeratin 7, negative for Hep Par 1, contained mucin, and had a Ki67 proliferation index of 8%. The morphology, immunophenotype, presence of mucin, and normal appearing bile ducts, as well as the increased Ki67 proliferation rate, were consistent with a bile duct adenoma with oxyphilic (oncocytic) change. Oncocytic tumors in the liver are rare; the first described in 1992. Only two bile duct adenomas with oncocytic change have been reported and neither of them had reported mucin production or the presence of normal appearing bile ducts within the lesion. 1. Introduction Bile duct adenomas are benign proliferations of intrahepatic bile ducts. These lesions are usually found under the liver capsule, often as an incidental finding during surgery or at autopsy. Oncocytic bile duct neoplasms in the liver are very rare. Of the cases described, most have been cystic neoplasms with the majority being malignant. A bile duct adenoma with oncocytic features is an exceptionally rare lesion. Only two cases have thus far been reported. Here we report a case of a bile duct lesion with oncocytic features that has elements of a bile duct adenoma, as well as oncocytic and mucinous features. 2. Case Summary A 61-year-old male recently diagnosed with colorectal adenocarcinoma presented for resection of his tumor. A CT scan showed the large mass in the descending and sigmoid colon with several small lymph nodes in the mesocolon suspicious for metastatic disease. No abnormalities were identified in the liver, gallbladder, pancreas, or spleen. During surgery, a small white 0.3？cm diameter nodule was noted on the surface of the right lobe of the liver. Thought to be possible metastatic disease, this lesion was excised and sent for pathologic evaluation. Pathologic examination showed a well-circumscribed lesion composed of cytologically bland cells arranged in tightly packed glands with fibrous stroma. There was associated moderate chronic inflammation. The tumor did not invade the adjacent liver (Figure 1). No cystic spaces, bile duct dilation, or inspissated bile was identified. The neoplastic cells had small,